Method for producing rigid polyurethane foams by means of graft polyhydric alcohols

ABSTRACT

The invention relates to a process for producing rigid polyurethane foams by reacting a) polyisocyanates with b) compounds having at least two hydrogen atoms reactive toward isocyanate groups, in the presence of c) catalysts, and d) blowing agents, where, among the compounds having at least two hydrogen atoms reactive toward isocyanate groups, there is at least one graft polyol present capable of preparation via in-situ polymerization of ethylenically unsaturated monomers in polyether alcohols.

The invention relates to a process for producing rigid polyurethanefoams by reacting polyisocyanates with compounds having at least twohydrogen atoms reactive toward isocyanate groups.

Rigid polyurethane foams have been known for a long time, and are mainlyused for thermal insulation, e.g. in refrigeration equipment, in tanksfor storing hot water, in long-distance heating pipes, or in theconstruction sector, for example in sandwich components. An overview ofthe production and use of rigid polyurethane foams is found by way ofexample in Kunststoff-Handbuch, Volume 7, Polyurethanes, 1^(st) Edition,1966, edited by Dr. R. vieweg and Dr. A. Höchtlen, 2^(nd) Edition 1983,edited by Dr. Günter Oertel, and 3^(rd) Edition 1993, edited by Dr.Günter Oertel, Carl Hanser Verlag, Munich, Vienna.

In the industrial production of rigid polyurethane foams, in particularof sandwich components, or the production of refrigeration equipment,the curing of the foams is particularly important.

Faster demolding times increase the capacity of existing productionlines with no need for investment in re-engineering of machinery. Whensandwich components are produced, faster curing permits higher twin-beltspeed, and thus higher output per unit of time.

The prior art discloses many ways of lowering demolding times.

For example, DE19630787 describes polyurethanes with improved curing viathe use of amine-containing polyols.

CA 2135352 describes polyurethanes with good demolding performance viathe use of a sucrose-started polyol.

According to JP 07082335, demolding is improved by using a mixture of1,3,5-tris(3-aminopropyl)hexahydro-s-triazine,pentamethyldiethylenetriamine, and bis(2-dimethylaminoethyl) ether ascatalysts.

According to JP 2001158815, good demolding is achieved via the use of amixture of aromatic polyester alcohols having a hydroxy value in therange from 405 to 500 mg KOH/g and a functionality of from 2 to 3 withpolyether alcohols based on TDA and propylene oxide and/or butyleneoxide having a hydroxy value of from 300 to 450 mg KOH/g and afunctionality of from 3 to 4.

According to JP 10101762, good demolding is achieved via asucrose-alkylene-oxide polyol with a molecular weight above 300 and afunctionality above 3.

According to JP 02180916, good demolding is achieved via an aromaticpolyesterol having a functionality of from 2.2 to 3.6 and a hydroxyvalue of from 200 to 550 mg KOH/g, prepared by esterifying an aromaticpolycarboxylic acid with diethylene glycol and with a trifunctionalalcohol.

For foams to be used in refrigeration equipment, and also for sandwichcomponents, operations therefore typically use modified catalysis and/oruse amine-started polyols which are highly functionalized or haveintrinsic reactivity and have a high hydroxy value, with the aim ofachieving a high level of crosslinking and thus faster curing.

The higher level of crosslinking frequently impairs the flowability ofthe reaction mixture, thus requiring more material in order to fill acavity (e.g. a mold or a refrigerator casing).

It is an object of the present invention to provide rigid polyurethanefoams which feature good curing and demoldability together with idealflow performance, while having good mechanical properties, in particulargood compressive strength.

We have found that this object is achieved, surprisingly, in that thepolyol component is composed to some extent or completely of graftpolyols.

The invention therefore provides a process for producing rigidpolyurethane foams by reacting

-   a) polyisocyanates with-   b) compounds having at least two hydrogen atoms reactive toward    isocyanate groups, in the presence of-   c) catalysts, and-   d) blowing agents,    which comprises the presence, among the compounds having at least    two hydrogen atoms reactive toward isocyanate groups, of at least    one graft polyol capable of preparation via in-situ polymerization    of ethylenically unsaturated monomers in polyether alcohols.

The rigid polyurethane foams produced according to the invention areusually closed-cell foams, and this means that the proportion of closedcells in the foam is at least 88%, preferably at least 95%.

The amount used of the graft polyols used according to the invention maybe up to 100% by weight. The preferred amount used is from 0.5 to 70% byweight, based in each case on component b.

The preferred amount of the graft polyols used when producingrefrigeration equipment is from 3 to 70% by weight, particularly from 3to 50% by weight, in particular from 3 to 35% by weight, based in eachcase on the weight of component b.

The preferred amount of the graft polyols used during the production ofsandwich components is from 0.5 to 35% by weight, with preference from0.5 to 25% by weight, and particularly from 1 to 20% by weight, based ineach case on the weight of component b.

The polyol mixtures comprising the graft polyols mostly have arelatively low storage stability. To render the systems processableduring the production of refrigeration equipment, continuous stirringthroughout the machine-foaming process is preferred.

For the production of sandwich components, it is preferable to use asuitable polyol, such as polypropylene glycols with a molar mass in therange from 300 to 1500 g/mol with the graft polyol to formulate anaddition component, which then has phase stability extending over weeksto months. This is then metered into the other components in the mixinghead. The storage stability of the polyol mixtures may be furtherincreased via the presence of conventional silicone stabilizers.

The graft polyols used for the process of the invention usually have ahydroxy value in the range from 20 to 120 mg KOH/g. They may be preparedby conventional and known processes.

The graft polyols used according to the invention, often also termedpolymer polyols, are dispersions of polymers, mostlyacrylonitrile-styrene copolymers, in a polyether alcohol.

Graft polymers may be prepared via free-radical polymerization of themonomers, preferably acrylonitrile, styrene, and also, whereappropriate, other monomers, or of a macromer, or of a moderator, usinga free-radical initiator, mostly azo compounds or peroxide compounds, ina continuous phase of polyetherol or polyesterol, often termed carrierpolyols.

Graft polyols are prepared by in-situ polymerization of acrylonitrile,styrene, or preferably mixtures of styrene and acrylonitrile, e.g. in aweight ratio of from 90:10 to 10:90, preferably from 70:30 to 30:70,using methods based on the data in German Patents 1111394, 1222669 (US3304273, 3383351, 3523093), 1152536 (GB 1040452), and 1152537 (GB987618).

Carrier polyols which may be used are compounds having at least afunctionality of from 2 to 8, preferably from 2 to 6, and an averagemolar mass of from 300 to 8000 g/mol, preferably from 300 to 5000 g/mol.The hydroxy value of the polyhydroxy compounds here is generally from 20to 160 and preferably from 28 to 56.

Macromers, also termed stabilizers, are linear or branched polyetherolswith molar masses ≧1000 g/mol, containing at least one terminal,reactive olefinic unsaturated group. The ethylenically unsaturated groupmay be introduced by subjecting a previously prepared polyol to reactionwith carboxylic anhydrides, such as maleic anhydride, with fumaric acid,with acrylate derivatives, with methacrylate derivatives, or else withisocyanate derivatives, such as 3-isopropenyl-1,1-dimethylbenzylisocyanates, or isocyanatoethyl methacrylates. Another route ispreparation of a polyol via alkoxidation of propylene oxide and ethyleneoxide, using starter molecules having hydroxy groups and ethylenicunsaturation. Examples of these macromers are described in the patentsU.S. Pat. No. 4,390,645, U.S. Pat. No. 5,364,906, EP 0 461 800, U.S.Pat. No. 4,997,857, U.S. Pat. No. 5,358,984, U.S. Pat. No. 5,990,232, WO01/04178, and U.S. Pat. No. 6,013,731.

During the free-radical polymerization, the macromers becomeincorporated into the copolymer chain. The result is formation of blockcopolymers having a polyether block and a polyacrylonitrile-styreneblock. These act as compatibilizer in the boundary between continuousand disperse phase, and suppress agglomeration of the graft polyolparticles. The proportion of the macromers is usually from 1 to 15% byweight, based on the total weight of the monomers used to prepare thegraft polyol.

Moderators, also termed chain transfer agents, are usually used in thepreparation of graft polyols. The use and the function of thesemoderators is described by way of example in U.S. Pat. No. 4,689,354, EP0 365 986, EP 0 510 533, and EP 0 640 633, EP 008 444, EP 0731 118 B1.Moderators reduce the molecular weight of the copolymers as they form,by subjecting the growing free radical to chain transfer. This reducesthe level of crosslinking between the polymer molecules, and thusaffects the viscosity and the dispersion stability of the graft polyols,and also their filterability. The proportion of the moderators isusually from 0.5 to 25% by weight, based on the total weight of themonomers used to prepare the graft polyol. Moderators usually used toprepare graft polyols are alcohols, such as 1-butanol, 2-butanol,isopropanol, ethanol, methanol, cyclohexane, toluene, mercaptans, suchas ethanethiol, 1-heptanethiol, 2-octanethiol, 1-dodecanethiol,thiophenol, 2-ethylhexyl thioglycolate, methyl thioglycolate, cyclohexylmercaptan, and also enol ether compounds, morpholine, andα(benzoyloxy)styrene.

To initiate the free-radical polymerization, use is usually made ofperoxide compounds or of azo compounds for example dibenzoyl peroxide,lauroyl peroxide, tert-amyl 2-ethylperoxyhexanoate, di-tert-butylperoxide, diisopropyl peroxide carbonate, tert-butyl2-ethylperoxyhexanoate, tert-butyl perpivalate, tert-butylperneodecanoate, tert-butyl perbenzoate, tert-butyl percrotonate,tert-butyl perisobutyrate, tert-butyl 1-methylperoxypropanoate,tert-butyl 2-ethylperoxypentanoate, tert-butyl peroxyoctanoate, anddi-tert-butyl perphthalate, 2,2′-azobis(2,4-dimethylvalereronitrile),2,2′-azobisisobutyronitrile (AIBN), 2,2′-azobis(dimethyl isobutyrate),2,2′-azobis(2-methylbutyronitrile) (AMBN),1,1′-azobis(1-cyclohexanecarbonitrile). The proportion of the initiatorsis usually from 0.1 to 6% by weight, based on the total weight of themonomers used to prepare the graft polyol.

For reasons associated with the reaction rate of the monomers, togetherwith the half-life time of the initiators, the free-radicalpolymerization to prepare graft polymers is usually carried out at from70 to 150° C. and at a pressure of up to 20 bar. Preferred reactionconditions for preparing graft polyols are from 80 to 140° C. at apressure of from atmospheric pressure to 15 bar.

In one embodiment of the process of the invention, the graft polyolsused for the process of the invention may be prepared using carrierpolyols whose properties are similar to those of conventional and knownflexible-foam polyether alcohols. These polyether alcohols mostly have afunctionality of from 2 to 8 and a hydroxy value in the range from 20 to100 mg KOH/g. They are prepared by an addition reaction of propyleneoxide, or of mixtures of ethylene oxide and propylene oxide, ontoH-functional starter substances, such as glycerol, trimethylolpropane,or glycols, e.g. ethylene glycol or propylene glycol. The catalysts usedfor the addition reaction of the alkylene oxides may comprise bases,preferably alkali metal hydroxides, or multimetal cyanide complexes,known as DMC catalysts. These graft polyols mostly have a hydroxy valuein the range from 10 to 70 mg KOH/g, with a solids content of from 35 to60%.

In one particular embodiment of the process of the invention, thecarrier polyols used comprise the type of polyether alcohols usuallyused to produce rigid polyurethane foams. These polyether alcoholsmostly have a functionality of from 2 to 8 and a hydroxy value in therange from 100 to 800 mg KOH/g. The starter substances used comprisepolyhydric alcohols, such as glycerol, trimethylolpropane, or sugaralcohols, such as sorbitol, sucrose, or glucose, or aliphatic amines,such as ethylenediamine, or aromatic amines, such as tolylenediamine(TDA), diphenylmethanediamine (MDA), or mixtures of MDA andpolyphenylene polymethylene polyamines. The alkylene oxides usedcomprise propylene oxide or mixtures of ethylene oxide and propyleneoxide. These graft polyols mostly have a hydroxy value in the range from60 to 150 mg KOH/g with a solids content of from 35 to 60%.

The crosslinking densities achievable for the polyurethane network arehigher when using these graft polyols than when using the known graftpolyols based on flexible-foam carrier polyols.

The graft polyols used according to the invention preferably have aparticle size of from 0.1 to 8 μm for the polymers, preferably from 0.2to 4 μm, with a particle size maximum at from 0.2 to 3 μm, preferably atfrom 0.2 to 2.0 μm. The solids content of the graft polyols is mostly inthe range from 10 to 60% by weight, based on the polyol.

Among these graft polyols, those whose use is preferred are based oncarrier polyols which are polyether alcohols which have a hydroxy valueof from 130 to 240 mg KOH/g and whose starter substance is vicinal TDA,which is subjected to an addition reaction with propylene oxide or witha mixture of from 5 to 12% by weight of ethylene oxide and from 88 to95% by weight of propylene oxide. The resultant graft polyols preferablyhave a hydroxy value of from 70 to 100 mg KOH/g and a solids content offrom 40 to 55% by weight, based on the entire graft polyol. The monomersused preferably comprise a mixture of acrylonitrile and styrene in aratio by weight of from 1:3 to 3:1, preferably 1:2.

In another embodiment of these graft polyols, the carrier polyols usedcomprise polyether alcohols which have a hydroxy value in the range from140 to 240 mg KOH/g and which are prepared by an addition reaction ofalkylene oxides, in particular propylene oxide or a mixture of propyleneoxide and ethylene oxide, onto conventional di- or trifunctional startersubstances, such as glycols, glycerol, and/or trimethylolpropane. Thesegraft polyols preferably have a hydroxy value in the range from 70 to110 mg KOH/g and a solids content of from 30 to 70% by weight, based onthe weight of the entire graft polyol.

In another preferred embodiment of the graft polyols used according tothe invention, the particle size distribution is bimodal, meaning thatthe particle size distribution curve has two maxima. One way ofpreparing these graft polyols is the mixing of graft polyols withmonomodal particle size distribution and with different particle size,in the appropriate ratio. In another method, however, the initial chargefor the reaction comprises a carrier polyol which is a polyol containingpolymers of olefinically unsaturated monomers. In this embodiment, too,the particle size is within the range described above.

The graft polyols used according to the invention may be prepared incontinuous processes or in batch processes. The synthesis of graftpolyols by both types of process is known, and there are manydescriptions of examples. For example, the synthesis of graft polyols bysemibatch processes is described in the following patents: EP 439755,U.S. Pat. No. 4,522,976, EP 163188, U.S. Pat. No. 5,830,944, EP 894812,U.S. Pat. No. 4,394,491 A, WO 87/03886, WO 97/44368, U.S. Pat. No.5,554,662. A specific form of the semibatch process is the semibatchseed process, in which the initial charge used for the reaction alsocomprises a graft polyol as seed, as described in EP 510533, EP 786480,and EP 698628, for example. The synthesis of graft polyols by acontinuous process is also known, and is described, inter alia, in WO00/59971, WO 99/31160, U.S. Pat. No. 5,955,534, U.S. Pat. No. 5,496,894,U.S. Pat. No. 5,364,906, U.S. Pat. No. 5,268,418, U.S. Pat. No.6,143,803, EP 0768324.

The following comments concern the other starting materials used for theprocess of the invention:

The organic polyisocyanates a) used preferably comprise aromaticpolyfunctional isocyanates.

Individual compounds which may be mentioned by way of example are:tolylene 2,4- and 2,6-diisocyanate (TDI) and the corresponding isomermixtures, diphenylmethane 4,4′-2,4′-, and 2,2′-diisocyanate (MDI) andthe corresponding isomer mixtures, mixtures of diphenylmethane 4,4′- and2,4′-diisocyanates, polyphenyl polymethylene polyisocyanates, mixturesof diphenylmethane 4,4′-, 2,4′-, and 2,2′-diisocyanates and polyphenylpolymethylene polyisocyanates (crude MDI), and mixtures of crude MDI andtolylene diisocyanates. The organic di- and polyisocyanates may be usedindividually or in the form of mixtures.

Use is also often made of what are known as modified polyfunctionalisocyanates, i.e. products obtained by chemical reaction of organic di-and/or polyisocyanates. Examples which may be mentioned are di- and/orpolyisocyanate containing isocyanurate groups and/or containing urethanegroups. The modified polyisocyanates may, where appropriate, be mixedwith one another or with unmodified organic polyisocyanates, e.g.diphenylmethane 2,4′-, or 4,4′-diisocyanate, crude MDI, 2,4-tolyleneand/or 2,6-diisocyanate.

Besides these, use may also be made of reaction products ofpolyfunctional isocyanates with polyhydric polyols, or else of mixturesof these with other di- or polyisocyanates.

An organic polyisocyanate which has proven particularly successful iscrude MDI with an NCO content of from 29 to 33% by weight and with aviscosity in the range from 150 to 1000 mPa·s at 25° C.

The compounds b which are used and which have at least two hydrogenatoms reactive toward isocyanate, and which may be used together withthe graft polyols used according to the invention, are in particularpolyether alcohols and/or polyester alcohols having hydroxy values inthe range from 100 to 1200 mg KOH/g.

The polyester alcohols used together with the graft polyols usedaccording to the invention are mostly obtained via condensation ofpolyhydric alcohols, preferably diols, having from 2 to 12 carbon atoms,preferably from 2 to 6 carbon atoms, with polybasic carboxylic acidshaving from 2 to 12 carbon atoms, such as succinic acid, glutaric acid,adipic acid, subaric acid, azelaic acid, sebacic acid,decanedicarboxylic acid, maleic acid, fumaric acid, or preferablyphthalic acid, isophthalic acid, terephthalic acid, or the isomericnaphthalenedicarboxylic acids.

The polyether alcohols used together with the graft polyols usedaccording to the invention mostly have a functionality of from 2 to 8,in particular from 3 to 8.

Use is particularly made of polyether polyols which are prepared byknown processes, for example via anionic polymerization of alkyleneoxides in the presence of catalysts, preferably of alkali metalhydroxides.

The alkylene oxides used are mostly ethylene oxide and/or propyleneoxide, preferably pure propylene 1,2-oxide.

Particular starter molecules which may be used are compounds having atleast 3, preferably from 4 to 8, hydroxy groups, or having at least twoprimary amino groups.

The starter molecules used and having at least 3, preferably from 4 to8, hydroxy groups are preferably trimethylolpropane, glycerol,pentaerythritol, sugar compounds, such as glucose, sorbitol, mannitol,and sucrose, polyhydric phenols, resols, e.g. oligomeric condensationproducts of phenol and formaldehyde, and Mannich condensates of phenolswith formaldehyde and with dialkanolamines, or else melamine.

The starter molecules used and having at least two primary amino groupsare preferably aromatic di- and/or polyamines, such as phenylenediamine,2,3-, 2,4-, 3,4-, and 2,6-tolylenediamine, and 4,4′-, 2,4′-, and2,2′-diaminodiphenylmethane, or else aliphatic di- and polyamines, e.g.ethylenediamine.

The polyether polyols have a functionality which is preferably from 3 to8, and hydroxy values which are preferably from 100 to 1200 mg KOH/g,and in particular from 240 to 570 mg KOH/g.

The use of difunctional polyols, such as polyethylene glycols, and/orpolypropylene glycols—with molecular weight in the range from 500 to1500—in the polyol component can adjust the viscosity of the polyolcomponent appropriately.

The compounds b having at least two hydrogen atoms reactive towardisocyanate also include any chain extenders and crosslinkers usedconcomitantly. The rigid PU foams may be produced with or withoutconcomitant use of chain extenders and/or of crosslinkers. Addition ofbifunctional chain extenders or of crosslinkers of functionality 3 orhigher, or where appropriate, of mixtures of these can proveadvantageous for modifying mechanical properties. The chain extendersand/or crosslinkers used preferably comprise alkanolamines and inparticular diols and/or triols with molecular weights which are smallerthan 400 and are preferably from 60 to 300.

The amount advantageously used of chain extenders, crosslinkers, ormixtures of these is advantageously from 1 to 20% by weight, preferablyfrom 2 to 5% by weight, based on the polyol component b.

Further data concerning the polyester alcohols and polyether alcoholsused, and also concerning their preparation, is found by way of examplein Kunststoffhandbuch, Volume 7 “Polyurethane”, edited by Günter Oertel,Carl-Hanser-Verlag Munich, 3^(rd) Edition, 1993.

The catalysts c used are in particular compounds which accelerate thereaction of the isocyanate groups with the groups reactive towardisocyanate groups.

These catalysts comprise strongly basic amines, e.g. secondary aliphaticamines, imidazoles, amidines, and also alkanolamines or organometalliccompounds, in particular organotin compounds.

If isocyanurate groups are also to be incorporated into the rigidpolyurethane foam, specific catalysts are needed for this purpose. Theusual isocyanurate catalysts used are metal carboxylates, in particularpotassium acetate and solutions thereof.

Depending on requirement, the catalysts may be used alone or in anydesired mixtures with one another.

The blowing agents d used may preferably be water, which reacts withisocyanate groups, with elimination of carbon dioxide. In combinationwith, or instead of, water it is also possible to use what are known asphysical blowing agents. These are compounds inert toward the startingcomponents, mostly liquid at room temperature, and vaporizing under theconditions of the urethane reaction. The boiling point of thesecompounds is preferably below 50° C. Physical blowing agents alsoinclude compounds which are gaseous at room temperature and which areintroduced into, or dissolved in, the starting components underpressure, examples being carbon dioxide, and low-boiling alkanes andfluoroalkanes.

The compounds are mostly selected from the group consisting of alkanesand cycloalkanes having at least 4 carbon atoms, dialkyl ethers, esters,ketones, acetals, fluoroalkanes having from 1 to 8 carbon atoms, andtetraalkylsilanes having from 1 to 3 carbon atoms in the alkyl chain, inparticular tetramethylsilane.

Examples which may be mentioned are propane, n-butane, iso- andcyclobutane, n-, iso-, and cyclopentane, cyclohexane, dimethyl ether,methyl ethyl ether, methyl butyl ether, methyl formate, acetone, andfluoroalkanes, where these compounds can be degraded in the troposphereand are therefore not injurious to the ozone layer, examples beingtrifluoromethane, difluoromethane, 1,1,1,3,3-pentafluorobutane,1,1,1,3,3-pentafluoropropane, 1,1,1,2-tetrafluoroethane, difluoroethane,and heptafluoropropane. It is preferable to use hydrocarbons containingno halogen atoms, in particular pentane, if appropriate mixed withpropane and with butanes. The physical blowing agents mentioned may beused alone or in any desired combinations with one another.

The process of the invention may, if required, be carried out in thepresence of flame retardants, and also of conventional auxiliariesand/or additives.

Flame retardants which may be used are phosphoric esters and/orphosphonic esters. It is preferable to use compounds which are notreactive toward isocyanate groups. Preferred compounds also includechlorine-containing phosphoric esters.

Typical representatives of this group of flame retardants are triethylphosphate, diphenyl cresyl phosphate, tris(chloropropyl) phosphate, anddiethyl ethanephosphonate.

Besides these, use may also be made of bromine-containing flameretardants. The bromine-containing flame retardants used are preferablycompounds having groups reactive toward the isocyanate group. Compoundsof this type are esters of tetrabromophthalic acid with aliphatic diols,and alkoxylation products of dibromobutenediol. Use may also be made ofcompounds derived from the series of brominated neopentyl compoundscontaining OH groups.

The auxiliaries and/or additives used comprise the substances known perse for this purpose, examples being surface-active substances, foamstabilizers, cell regulators, fillers, pigments, dyes, flame retardants,hydrolysis stabilizers, antistats, and agents with fungistatic andbacteriostatic action.

Further details concerning the starting materials, blowing agents,catalysts, and auxiliaries and/or additives used to carry out theprocess of the invention are found by way of example inKunststoffhandbuch, Volume 7, “polyurethane” Carl-Hanser-Verlag, Munich,1^(st) Edition, 1966, 2^(nd) Edition, 1983 and 3^(rd) Edition, 1993.

To produce the rigid polyurethane foams, the polyisocyanates a and thecompounds b having at least two hydrogen atoms reactive 15 towardisocyanate groups are reacted in amounts such that the isocyanate indexis in the range from 100 to 220, preferably from 115 to 195. The rigidpolyurethane foams may be produced batchwise or continuously with theaid of known mixing apparatus.

A higher index, preferably up to 350, may also be used when producingpolyisocyanurate foams.

The rigid PU foams of the invention are usually produced by thetwo-component process. This process prepares a mixture using thecompounds b having at least two hydrogen atoms reactive towardisocyanate groups, the flame retardants, the catalysts c, the blowingagents d, and also the other auxiliaries and/or additives, to give whatis known as a polyol component, and reacts this with the polyisocyanatesor mixtures of the polyisocyanates and, where appropriate, blowingagents, also termed the isocyanate component.

The starting components are mostly mixed at from 15 to 35° C.,preferably from 20 to 30° C. Using high- or low-pressure meteringmachinery, the reaction mixture may be poured into closed support molds.An example of the use of this technology is the batchwise manufacture ofsandwich components.

The reaction mixture may also be injected or poured without constraintonto surfaces or into open cavities. This process can be used toinsulate roofs or complicated vessels in situ.

Another preferred embodiment of the process of the invention iscontinuous mixing of the isocyanate component with the polyol componentto produce sandwich components or insulation components on twin-beltsystems. The usual method with this technology is to meter the catalystsand the blowing agents into the polyol component by way of othermetering pumps. The components used here may be divided into up to 8separate components. Using the two-component process as a basis, thefoaming formulations may readily be recalculated for the processing ofmulticomponent systems.

As stated above, the rigid polyurethane foams produced by the process ofthe invention have ideal processing properties, and in particular givegood curing. Surprisingly, the rigid polyurethane foams produced by theprocess of the invention have reduced tendency toward cavitation.

The amount of isocyanate used to produce the foams can be reduced, sincethe hydroxy value of the graft polyols used is lower than that ofconventional rigid-foam polyether alcohols.

The examples below are intended to provide an illustration of theinvention in greater detail.

Test Methods

-   1) The bolt test was used to determine curing. In this test, 2, 3,    and 4 minutes after mixing of the components in a polystyrene beaker    a steel bolt with a spherical head of radius 10 mm was pressed to a    depth of 10 mm into the resultant foam cushion, using a    tensile/pressure testing machine. The maximum force required for    this in N is a measure of the curing of the foam. The data given in    each case is the total of the maximum forces measured after 2, 3 and    4 minutes.-   2) Flowability was determined using the hose test. For this, 100 g    of the reaction mixture obtained by mixing of the components was    poured into a plastic hose of diameter 45 mm, and the hose is    sealed. The length of the flow path within the plastic hose in cm is    a measure of flowability.-   3) Thermal conductivity was determined to DIN 52 616-77. The test    specimens were produced by pouring the polyurethane reaction mixture    into a mold of dimensions 22.5×22.5×22 cm (10% overfilling) and    cutting a test specimen of dimensions 20×20×5 cm from the center    after some hours.-   4) Compressive strength was determined to DIN 52 421/DIN EN ISO 604.-   5) Visual assessment of foam structure/fine cells in foam. 1: very    fine-celled; 2: fine-celled; 3: slight degree of coarse-celled    structure; 4: coarse-celled. The proportion of closed cells was    determined to ISO 4590.-   6) Visual assessment of tendency of sandwich components to develop    base-surface defects or to develop cavities. 1: very smooth surface,    no cavities/base-surface defects of any kind on underside of    sandwich component; 2: very few slight cavities/base-surface defects    on underside of sandwich component; 3: some cavities/base-surface    defects on underside of sandwich component; 4: substantial    base-surface defects across entire surface of underside of sandwich    component-   7) Assessment of curing of sandwich components at end of belt: 1:    minimal change in component thickness after 24 hours; 2: slight    change in component thickness after 24 hours; 3: marked change in    component thickness after 24 hours-   8) Fire performance was determined in the DIN 4102 small burner test    Preparation of Graft Polyols    Semi-Batch Preparation of Graft Polyols

Semi-batch preparation of graft polyols took place in a 2-literautoclave equipped with 2-stage stirrer, internal cooling coil, andelectrical heating jacket. Prior to start of the reaction, the reactorwas charged with a mixture of carrier polyol and macromer, flushed withnitrogen, and heated to the synthesis temperature of 125 or 130° C. Forsome syntheses, a graft polyol (polyol 28) was also added as seed to theinitial charge for the reaction, alongside the carrier polyol and themacromer.

The remainder of the reaction mixture, composed of further carrierpolyol, initiator, the monomers, and the reaction moderator, formed aninitial charge in at least two feed vessels. The graft polyols weresynthesized by transferring the raw materials from the feed vessels at aconstant metering rate by way of a static in-line mixer into thereactor. The feed time for the monomer-moderator mixture was 150 or 180minutes, while the polyol-initiator mixture was metered into the reactorduring 165 or 195 minutes. After a further period of from 10 to 30minutes of continued reaction time at reaction temperature, the crudegraft polyol was transferred by way of the basal discharge valve into aglass flask. The product was then freed from unreacted monomers andother volatile components at 135° C. in vacuo (<0.1 mbar). The finalproduct was finally stabilized with antioxidants. TABLE 0 Graft polyolsfrom semi-batch preparation Polyol 20 Polyol 21 Polyol 23 Polyol 24Polyol 27 Polyol 28 Reaction conditions Temperature (° C.) 125 120 130125 125 125 Initial pressure (bar) 0 0 0 3 3 1 Initial reactor chargeCarrier Polyol (g) Polyol 13 Polyol 14 Polyol 13 Polyol 12 Polyol 15Polyol 15 214.17 275.2 336.70 356.07 356.07 336.58 Macromer (g) Polyol16 Polyol 16 Polyol 16 Polyol 16 Polyol 16 Polyol 16 27.36 22.8 18.2423.38 23.38 18.14 Seed (g) — — Polyol 28 Polyol 28 Polyol 28 — 60.00122.10 122.10 Feed stream 1 Acrylonitrile (g) 239.98 199.98 159.98205.02 205.02 159.98 Styrene (g) 480.02 400.02 320.02 410.10 410.10320.02 n-Dodecanethiol (g) 7.27 6.06 4.85 6.46 6.46 5.23 Feed time (min)180 180 150 150 150 150 Feed stream 2 Carrier polyol (g) Polyol 13Polyol 14 Polyol 13 Polyol 12 Polyol 15 Polyol 15 227.60 292.23 357.81378.4 378.4 357.68 Initiator (g) Initiator 1 Initiator 2 Initiator 1Initiator 1 Initiator 1 Initiator 3 3.60 3.48 2.40 2.86 2.86 2.36 Feedtime (min) 195 195 165 165 165 165Continuous Preparation of Polyol 22

For continuous preparation of graft polyols under superatmosphericpressure, use was made of a 300 ml stirred reactor with continuous in-and outflow. Prior to the start of the reaction, the reactor was filledwith polyol 12 or graft polyol from the preceding synthesis, and heatedto the synthesis temperature of 133° C. The reaction mixture wasprovided in two feed vessels, and pumped into the reactor, using thefeed rates given. Feed stream 1 Feed rate: 14.54 g/min Acrylonitrile449.96 g Styrene 900.05 g Isopropanol 202.50 g Feed stream 2 Feed time:15.46 g/min Polyol 12 1578.45 g Polyol 16 60.75 g Initiator 2 10.80 g

Prior to entry into the reactor, the two feed streams were combined byway of an in-line static mixer. The product obtained during the initialphase was discarded. Continuous operating conditions are usuallyachieved when the turnover factor has reached 10, corresponding to about3000 ml.

The reaction mixture was pumped into the reactor by way of an aperturein the base, and intimately mixed by stirring (1500 rpm) with thematerial previously introduced thereto, and was discharged from thereactor by way of a controllable spring-loaded retention valve at thehead of the reactor. The pressure in the reactor was maintained at from4 to 10 bar, the reaction temperature being from 140 to 145° C. Afterdischarge from the reactor, the crude graft polyol, now at atmosphericpressure was collected in a glass flask. The product was then freed fromunreacted monomers and other volatile compounds at 135° C. in vacuo(<0.1 mbar). The final product was finally stabilized, usingantioxidants.

EXAMPLES 1 TO 8 AND COMPARATIVE EXAMPLES 1 TO 4 Comparative Example 1Rigid Foam for Use in Refrigeration Equipment; Manual Foaming

A polyol component was prepared by mixing 54.4 parts by weight of apolyether alcohol based on sorbitol and propylene oxide and having ahydroxy value of 490 mg KOH/g (polyol 1), 25.0 parts by weight of apolyether alcohol based on sucrose, glycerol, and propylene oxide,hydroxy value: 490 mg KOH/g (polyol 2), 0.8 part by weight of glycerol,1.7 parts by weight of silicone stabilizer L 6900 from Crompton, 1.3parts by weight of water, 0.7 part by weight ofN,N-dimethylcyclohexylamine, 1.1 parts by weight of Lupragen® N301, BASFAktiengesellschaft, and 0.6 part by weight of Lupragen® N600, BASFAktiengesellschaft, and 14 parts by weight of cyclopentane.

100 parts by weight of polyol component were mixed with 150 parts 40 byweight of a mixture of diphenylmethane diisocyanate and polyphenylenepolymethylene polyisocyanate having a NCO content of 31.5% by weight anda viscosity of 200 mPas (25° C.). This corresponds to an index of 132.The mixture was mixed using a Vollrath agitator with a maximum rotationrate of 1500 rpm, and then permitted to cure without restraint.

The envelope density of the resultant foam was 30 g/l and its thermalconductivity was 20.5 mW/mK. Its curing level, determined from theaverage of the impression hardness measurements after 2, 3, and 4minutes, was 135 N. The proportion of open cells was 10%.

Example 1

(Rigid Foam for Use in Refrigeration Equipment)

The procedure was as in comparative example 1, but polyol 2 was reducedby 25 parts by weight, these being replaced by 25 parts by weight of agraft polyol having a hydroxy value of 60.2 mg KOH/g, a solids contentof 60% by weight, and a viscosity of 30 000 mPas at 25° C., prepared bypolymerizing acrylonitrile and styrene in situ in a ratio of 1:2 byweight in a carrier polyol based on trimethylolpropane and propyleneoxide and having a hydroxy value of 160 mg KOH/g (polyol 20).

100 parts by weight of polyol component were mixed with 113 parts byweight of a mixture of diphenylmethane diisocyanate and polyphenylenepolymethylene polyisocyanate having an NCO content of 31.5% by weightand a viscosity of 200 mPas (25° C.). This corresponds to an index of132. The mixture was mixed using a Vollrath agitator with a maximumrotation rate of 1500 rpm, and then permitted to cure without restraint.

The envelope density of the resultant foam was 30 g/l and its thermalconductivity was 19.2 mW/mK. Its curing level, determined from theaverage of the impression hardness measurements after 2, 3, and 4minutes, was 177 N. The proportion of open cells was 9%.

Examples 2 to 6 and comparative example 2 used the same process. The rawmaterials used are shown in table 1, as are the foam propertiesdetermined.

Comparative Example 2

(Rigid Foam for Use in Refrigeration Equipment; Machine Foaming)

A polyol component was prepared by mixing 20 parts by weight of polyol1, 35.6 parts by weight of a polyether alcohol based on sucrose,pentaerythritol, diethylene glycol, and propylene oxide having a hydroxyvalue of 400 mg KOH/g (polyol 3), 30 parts by weight of a polyetheralcohol made from vicinal tolylenediamine, ethylene oxide, and propyleneoxide, having a hydroxy value of 45 400 mg KOH/g (polyol 4), 7 parts byweight of castor oil, 3 parts by weight of silicone stabilizer TegostabB 8467 from Degussa, 2.3 parts by weight of water, 0.7 part by weight ofdimethylcyclohexylamine, 0.7 part by weight of Lupragen® N301, BASFAktiengesellschaft, 0.7 part by weight of Dabco® T from Air Products,and 14 parts by weight of cyclopentane.

100 parts by weight of polyol component were mixed with 134 parts byweight of a mixture of diphenylmethane diisocyanate and polyphenylenepolymethylene polyisocyanate having an NCO content of 31.5% by weightand a viscosity of 200 mPas (25° C.) in a Puromat® HD 30 high-pressurefoaming machine (Elastogran GmbH). This corresponds to an index of 122.The reaction mixture was injected into a mold of dimensions 200×20×5 cm(Bosch lance) or 40×70×9 cm, where it was foamed.

The properties of the foam are given in table 2.

Example 4

(Rigid Foam for Use in Refrigeration Equipment)

The procedure was as in comparative example 2, but polyol 3 was reducedby 25 parts by weight, these being replaced by 25 parts by weight of agraft polyol having a hydroxy value of 20 mg KOH/g, a solids content of45% by weight, and a viscosity of 8000 mPas at 25° C., prepared bypolymerizing acrylonitrile and styrene in situ in a ratio of 1:2 byweight in a carrier polyol based on glycerol, propylene oxide, andethylene oxide, having a hydroxy value of 35 mg KOH/g. (Polyol 22).

100 parts by weight of polyol component were mixed with 114 parts byweight of a mixture of diphenylmethane diisocyanate and polyphenylenepolymethylene polyisocyanate with a NCO content of 31.5% by weight and aviscosity of 200 mPas (25° C.). This corresponds to an index of 125. Thereaction mixture was injected into a mold of dimensions 200×20×5 cm or40×70×9 cm, where it was foamed.

Examples 7 and 8 and comparative examples 3 and 4 used the same process.The raw materials used are shown in table 2, as are the foam propertiesdetermined. TABLE 1 Production of foams (manual foaming) ComparativeComparative example 1 Example 1 Example 2 Example 3 ex. 2 Example 4Example 5 Example 6 Polyol 1 54.4 54.4 54.4 54.4 20 20 20 20 Polyol 2 2515 Polyol 3 35.6 10.6 25.6 10.6 Polyol 4 30 30 30 30 Glycerol 0.8 0.80.8 0.8 Polyol 20 25 10 Polyol 21 25 25 Polyol 22 25 10 Castor oil 7 7 77 Stabilizer 1 1.7 1.7 1.7 1.7 Stabilizer 2 3 3 3 3 Water 1.3 1.3 1.31.3 2.3 2.3 2.3 2.3 Catalyst 1 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 Catalyst2 1.1 1.1 1.1 1.1 0.7 0.7 0.7 0.7 Catalyst 3 0.6 0.6 0.6 0.6 Catalyst 40.7 0.7 0.7 0.7 Cyclopentane 14 14 14 14 14 14 14 14 Mixing ratio 100:150 113 144 125 134 114 127 117 Index 132 132 132 132 125 125 125 125Cream time [s] 8 7 8 7 8 9 9 8 Fiber time [s] 50 49 52 48 53 58 57 56Envelope density 30 30 29 30 28 28 27 28 [g/l] Open cells [%] 10 9 10 119 8 10 11 Bolt test [N/mm²] 135 177 152 172 64 101 85 99 Thermalconductivity 20.5 19.2 19.9 19.4 19.1 20.3 19.8 20.1 [mW/mK]

TABLE 2 Production of foams (machine foaming) Comparative ComparativeComparative example 2 Example 4 example 3 Example 7 example 4 Example 8Polyol 1 20 20 20 20 20 20 Polyol 3 35.6 10.6 35.6 10.6 35.6 10.6 Polyol4 30 30 30 30 30 30 Polyol 22 25 25 25 Castor oil 7 7 7 7 7 7 Stabilizer2 3 3 3 3 3 3 Water 2.3 2.3 2.3 2.3 2.3 2.3 Catalyst 1 0.7 0.7 0.7 0.70.7 0.7 Catalyst 2 0.7 0.7 0.7 0.7 0.7 0.7 Catalyst 4 0.7 0.7 0.7 0.70.7 0.7 Cyclopentane 14 14 9.8 9.8 12.7 12.7 Isopentane 4.2 4.2Isobutane 1.3 1.3 Mixing ratio 100: 134 114 134 114 134 114 Index 132132 132 132 132 132 Fiber time [s] 44 41 47 45 47 44 Free-foamedenvelope density 22.0 22.5 20.8 20.7 20.2 20.6 [g/l] Minimum apparentdensity 31.8 31.2 29.5 28.3 28.8 28.3 [g/l] Flow factor (min. apparent1.45 1.39 1.41 1.37 1.42 1.39 density/free envelope density) Open cells[%] 6 7 5 8 6 6 Thermal conductivity 19.3 19.2 19.4 19.2 19.2 19.1[mW/mK] Compressive strength (RD 35) 0.135 0.12 0.13 0.12 0.135 0.12[N/mm²] Continued expansion after 24 h, 91.3 90.5 91.1 90.6 91.1 90.410% overpack [mm]

EXAMPLES 10 TO 27 AND COMPARATIVE EXAMPLES 5 TO 7

(Production of Sandwich Components)

A polyol component was prepared from the starting materials listed intables 3, 4 and 5, and reacted in the stated mixing ratio on a twin-beltsystem with a mixture of diphenylmethane diisocyanate and polyphenylenepolymethylene polyisocyanate with an NCO content of 31.0% by weight anda viscosity of 520 mPas (25° C.) to produce a sandwich component with athickness of from 80 to 120 mm.

Tables 3 to 5 list the raw materials used and the properties of thesandwich components. TABLE 3 Compar- ative Example Example ExampleExample Example Example Example example 5 10 11 12 13 14 15 16 Polyol 220 20 20 20 20 20 20 20 Polyol 5 18.5 18.5 18.5 18.5 18.5 18.5 18.5 18.5Polyol6 16 16 16 16 16 16 16 16 Polyol 7 20 20 20 20 20 20 20 20 Polyol810 10 10 10 10 10 10 10 Glycerol 2 2 2 2 2 2 2 2 Dipropylene glycol 0.20.2 0.2 0.2 0.2 0.2 0.2 0.2 Polyol2l 5 Polyol 23 10 5 Polyol 24 5 Polyol25 5 4.5 Polyol 26 5 Flame retardant 1 12 12 12 12 12 12 12 12Stabilizer 3 1 1 1 1 1 1 1.5 1 Catalyst 1 4.6 4.6 4.6 4.6 4.6 4.6 4.64.6 Water 2.53 2.53 2.53 2.53 2.53 2.53 2.53 2.53 n-Pentane 7.0 7.0 7.07.0 7.0 7.0 7.0 7.0 Mixing ratio 100: 119 119 119 119 119 119 119 119Cream time [s] 15 15 14 15 16 15 15 16 Fiber time [s] 45 45 44 45 46 4445 47 Envelope density [g/l] 42 41 42 43 42 44 42 43 Component thickness80 80 80 80 80 80 80 80 [mm] Bolt test [N] 168 206 225 211 222 230 215221 Open cells [%] 8 10 10 8 11 10 9 9 Fire performance B3 B3 B3 B3 B3B3 B3 B3 (DIN 4102) Curing at end of belt 3 2 1 1-2 1-2 1-2 1-2 1-2Cavitation frequency 3 2-3 1-2 2 1-2 1-2 2 2 Foam structure 2 2 2 2 2 22 2

TABLE 4 Comparative example 6 Example 17 Example 18 Example 19 Example20 Example 21 Example 22 Polyol 2 51.15 51.15 51.15 51.15 51.15 51.1551.15 Polyol 9 5 5 5 5 5 5 5 Glycerol 4.5 4.5 4.5 4.5 4.5 4.5 4.5Dipropylene glycol 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Polyol 20 5 Polyol 23 5Polyol 24 5 Polyol 25 5 Polyol 27 10 5 Flame retardant 1 20 20 20 20 2020 20 Flame retardant 2 5 5 5 5 5 5 5 Flame retardant 3 12.5 12.5 12.512.5 12.5 12.5 12.5 Stabilizer 4 1.3 1.3 0.5 0.5 0.5 0.5 0.5 Stabilizer5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Catalysts 3.1 3.1 3.1 3.1 3.1 3.1 3.1Water 2.55 2.55 2.55 2.55 2.55 2.55 2.55 n-Pentane 6.0 6.0 6.0 6.0 6.06.0 6.0 Mixing ratio 100: 126 126 126 126 126 126 126 Cream time [s] 1716 17 18 18 17 17 Fiber time [s] 45 44 45 46 46 44 45 Envelope density[g/l]/ 40 41 40 40 39 40 40 Component thickness [mm] 120 120 120 120 120120 120 Open cells [%] 8 9 11 10 10 9 11 Bolt test [N] 120 185 194 201203 231 206 Fire performance (DIN 4102) B2 B2 B2 B2 B2 B2 B2 Curing atend of belt 3 2 1-2 1-2 1-2 1 1-2 Cavitation frequency 3 2 1-2 1-2 2 1-21-2 Foam structure 2 2 2 2 22 2 2

TABLE 5 Comparative example 7 Example 23 Example 24 Example 25 Example26 Example 27 Polyol 11 31.14 31.14 31.14 31.14 31.14 31.14 Polyol 1238.47 38.47 38.47 38.47 38.47 38.47 Polyol 24 2 Polyol 25 5 4.5 Polyol26 5 Polyol 27 5 Dipropylene glycol 20.25 20.25 20.25 20.25 20.25 20.25Ethylene glycol 3.3 3.3 3.3 3.3 3.3 3.3 Stabilizer 6 3.12 3.12 3.12 3.623.12 3.12 Catalyst 2 0.32 0.32 0.32 0.32 0.32 0.32 Catalyst 6 2.93 2.932.93 2.93 2.93 2.93 Water 0.47 0.47 0.47 0.47 0.47 0.47 Cyclopentane 1717 17 17 17 17 Mixing ratio 100: 300 300 300 300 300 300 Cream time [s]18 17 18 16 19 18 Fiber time [s] 29 30 29 29 31 30 Envelope density[g/l] 69 70 69 69 69 70 Component thickness [mm] 80 80 80 80 80 80 Opencells [%] 7 8 6 7 5 7 Fire performance (DIN 4102) B3 B3 B3 B3 B3 B3Curing at end of belt 3 2-3 1-2 1-2 1-2 1 Cavitation frequency 3 2-3 1-21 1 1-2 Foam structure 2 2 2 2 2 2Raw Materials Used:

-   Polyol 1: Polyether alcohol based on sorbitol, propylene oxide,    hydroxy value: 500 mg KOH/g-   Polyol 2: Polyether alcohol based on sucrose, glycerol, and    propylene oxide, hydroxy value: 490 mg KOH/g-   Polyol 3: Polyether alcohol based on sucrose, pentaerythritol,    diethylene glycol, and propylene oxide, hydroxy value: 400 mg KOH/g-   Polyol 4: Polyether alcohol made from vicinal tolylenediamine,    ethylene oxide, and propylene oxide, hydroxy value:-   400 mg KOH/g-   Polyol 5: Polyether alcohol based on sucrose, diethylene glycol, and    propylene oxide, hydroxy value: 440 mg KOH/g-   Polyol 6: Polyether alcohol based on propylene glycol and propylene    oxide, hydroxy value: 105 mg KOH/g-   Polyol 7: Polyether alcohol based on sorbitol and propylene oxide,    hydroxy value: 340 mg KOH/g-   Polyol 8: Polyester alcohol based on industrial dimer fatty acid,    glycerol, hydroxy value: 400 mg KOH/g-   Polyol 9: Polyether alcohol based on ethylenediamine and propylene    oxide, hydroxy value: 770 mg KOH/g-   Polyol 10: Polyether alcohol based on propylene glycol and propylene    oxide, hydroxy value: 250 mg KOH/g-   Polyol 11: Polyester alcohol prepared from adipic acid, phthalic    anhydride, oleic acid, and 1,1,1-trimethylolpropane, hydroxy value    385 mg KOH/g-   Polyol 12: Polyether alcohol based on glycerol, ethylene oxide, and    propylene oxide, hydroxy value: 35 mg KOH/g-   Polyol 13: Polyether alcohol based on trimethylolpropane and    propylene oxide, hydroxy value: 160 mg KOH/g-   Polyol 14: Polyether alcohol based on tolylenediamine, ethylene    oxide, and propylene oxide, hydroxy value: 160 mg KOH/g-   Polyol 15: Polyether alcohol based on glycerol, ethylene glycol,    ethylene oxide, and propylene oxide, hydroxy value: 48 mg KOH/g-   Polyol 16: Monofumarate ester having a hydroxy value of 18.8 mg    KOH/g and a viscosity of 7400 mPas, prepared by reacting maleic    anhydride with a polyol based on trimethylolpropane, propylene    oxide, and ethylene oxide, having a hydroxy value of 26.6 mg KOH/g.-   Polyol 20: Graft polyol having a hydroxy value of 60.2 mg KOH/g, a    solids content of 60% by weight, and a viscosity of 30 000 mPas at    25° C., prepared by polymerizing acrylonitrile and styrene in situ    in a ratio of 1:2 by weight in a carrier polyol based on    trimethylolpropane and propylene oxide, hydroxy value: 160 mg KOH/g-   Polyol 21: Graft polyol having a hydroxy value of 77 mg KOH/g, a    solids content of 52% by weight, and a viscosity of 42 000 mPas at    25° C., prepared by polymerizing acrylonitrile and styrene in situ    in a ratio of 1:2 by weight in a carrier polyol based on vicinal    tolylenediamine, ethylene oxide, and propylene oxide, hydroxy value:    160 mg KOH/g-   Polyol 22: Graft polyol having a hydroxy value of 20 mg KOH/g, a    solids content of 45% by weight, and a viscosity of 9000 mPas at 25°    C., prepared by polymerizing acrylonitrile and styrene in situ in a    ratio of 1:2 by weight in a carrier polyol based on glycerol,    propylene oxide, and ethylene oxide, hydroxy value: 35 mg KOH/g-   Polyol 23: Graft polyol having a hydroxy value of 91 mg KOH/g, a    solids content of 41% by weight, and a viscosity of 45 3000 mPas at    25° C., prepared by polymerizing acrylonitrile and styrene in situ    in a ratio of 1:2 by weight in a carrier polyol based on    trimethylolpropane and propylene oxide, hydroxy value: 160 mg KOH/g-   Polyol 24: Graft polyol having a hydroxy value of 20 mg KOH/g, a    solids content of 45% by weight, and a viscosity of 9000 mPas at 25°    C., prepared by polymerizing acrylonitrile and styrene in situ in a    ratio of 1:2 by weight in a carrier polyol based on    trimethylolpropane and propylene oxide, hydroxy value: 35 mg KOH/g-   Polyol 25: Mixture of 2 parts by weight of polyol 22 and 3 parts by    weight of polyol 10-   Polyol 26: Mixture of 2 parts by weight of polyol 22 and 3 parts by    weight of polyol 6-   Polyol 27: Graft polyol having a hydroxy value of 26 mg KOH/g, a    solids content of 45% by weight, and a viscosity of 6000 mPas at 25°    C., prepared by polymerizing acrylonitrile and styrene in situ in a    ratio of 1:2 by weight in a carrier polyol based on glycerol,    ethylene glycol, and propylene oxide and ethylene oxide, hydroxy    value: 48 mg KOH/g-   Polyol 28: Graft polyol having a hydroxy value of 28.4 mg KOH/g, a    solids content of 41% by weight, and a viscosity of 4500 mPas at 25°    C., prepared by polymerizing acrylonitrile and styrene in situ in a    ratio of 1:2 by weight in a carrier polyol based on glycerol and    monoethylene glycol, ethylene oxide and propylene oxide, hydroxy    value: 48 mg KOH/g-   Flame retardant 1: Trischloropropyl phosphate-   Flame retardant 2: Diethyl ethanephosphonate-   Flame retardant 3: Ixol® B251, Solvay AG-   Stabilizer 1: L6900, Crompton Corp.-   Stabilizer 2: Tegostab® B8467, Degussa AG-   Stabilizer 3: OS340, Bayer AG-   Stabilizer 4: Tegostab® B8466, Degussa AG-   Stabilizer 5: Dabco® DC5103, Air Products-   Stabilizer 6: 1:1 mixture of Tegostab® B8461 and Tegostab® B8409,    Degussa AG-   Catalyst 1: N,N-Dimethylcyclohexylamine-   Catalyst 2: Lupragen® N301, BASF Aktiengesellschaft-   Catalyst 3: Lupragen® N600, BASF Aktiengesellschaft-   Catalyst 4: Dabco® T, Air Products-   Catalyst 5: KX315, Elastogran GmbH-   Catalyst 6: 47% strength solution of potassium acetate in ethylene    glycol-   Initiator 1: Trigonox® 121, Akzo Nobel Chemikals GmbH-   Initiator 2: Vazo® 67, Du Pont de Nemours GmbH-   Initiator 3: Wako® V 601, Wako Chemicals GmbH

1. A process for producing closed-celled rigid polyurethane foams byreacting a) crude MDI having an NCO content of from 29 to 33% by weightand a viscosity at 25° C. in the range from 150 to 1000 mPa·s with b)compounds having at least two hydrogen atoms reactive toward isocyanategroups, in the presence of c) catalysts, and d) blowing agents, whichcomprises the presence, among the compounds having at least two hydrogenatoms reactive toward isocyanate groups, of at least one graft polyolcapable of preparation via in-situ polymerization of ethylenicallyunsaturated monomers in polyether alcohols.
 2. A process as claimed inclaim 1, wherein the amount used of the graft polyols is up to 100% byweight, based on component b.
 3. A process as claimed in claim 1,wherein the amount used of the graft polyols is from 0.5 to 70% byweight, based in each case on component b.
 4. A process as claimed inclaim 1, wherein the amount used of the graft polyols during theproduction of rigid polyurethane foams for use in refrigerationequipment is from 3 to 70% by weight, based on component b.
 5. A processas claimed in claim 1, wherein the amount used of the graft polyolsduring the production of rigid polyurethane foams for use in sandwichcomponents is from 0.5 to 35% by weight, based on component b.
 6. Aprocess as claimed in claim 1, wherein the graft polyols have a hydroxyvalue in the range from 20 to 210 mg KOH/g.
 7. A process as claimed inclaim 1, wherein the graft polyol particle distribution has a maximum atfrom 0.1 to 8 μm.
 8. A process as claimed in claim 1, wherein the graftpolyols have bimodal particle size distribution with two clearlyseparated maxima for the polymers.
 9. A process as claimed in claim 1,wherein the graft polyols are prepared by in-situ polymerization ofethylenically unsaturated monomers in polyether alcohols having afunctionality of from 2 to 8 and having a hydroxy value in the rangefrom 100 to 800 mg KOH/g, obtainable by an addition reaction of alkyleneoxides onto H-functional starter substances, the starter substanceshaving been selected from the group consisting of polyfunctionalalcohols, sugar alcohols, aliphatic amines, and aromatic amines.
 10. Aprocess as claimed in claim 1 wherein the graft polyols can be preparedby in-situ polymerization of ethylenically unsaturated monomers inpolyether alcohols which are obtained by an addition reaction ofalkylene oxides onto tolylenediamine, using basic catalysis.
 11. Aprocess as claimed in claim 1, wherein the graft polyols can be preparedby in-situ polymerization of ethylenically unsaturated monomers inpolyether alcohols which are obtained by an addition reaction ofalkylene oxides onto trimethylolpropane, using basic catalysis orcatalysis by multimetal cyanide complexes.
 12. A rigid polyurethane foamproduced by the process of claim
 1. 13. A graft polyol capable ofpreparation by in-situ polymerization of ethylenically unsaturatedmonomers in polyether alcohols having a hydroxy value in the range from100 to 600 mg KOH/g, obtainable by an addition reaction of alkyleneoxides onto H-functional starter substances, the starter substanceshaving been selected from the group consisting of polyfunctionalalcohols, sugar alcohols, aliphatic amines, and aromatic amines.
 14. Agraft polyol as claimed in claim 13, by in-situ polymerization ofethylenically unsaturated monomers in polyether alcohols having ahydroxy value in the range from 140 to 240 mg KOH/g, which are obtainedby an addition reaction of alkylene oxides onto tolylenediamine.
 15. Agraft polyol as claimed in claim 13, by in-situ polymerization ofethylenically unsaturated monomers in polyether alcohols having ahydroxy value in the range from 140 to 240 mg KOH/g, which are obtainedby an addition reaction of alkylene oxides onto trimethylolpropane.